1. Field of the Invention
The present invention generally relates to an image processing apparatus, an image forming apparatus using the image processing apparatus, an image processing method, a computer-readable recording medium storing a program of the method, and an image forming method using the image processing method.
2. Description of the Related Art
In a general-use laser printer, in many cases, the number of tones by which dots of image data are expressed is insufficient to obtain a natural image. Therefore, a pseudo continuous tone process is applied to original image data.
When image data having multi tones are output by application of the pseudo continuous tone process, input image data are processed mainly in the following processes. That is, the processes are an image data writing process onto a photoconductor to form a latent image, a developing process of the latent image on the photoconductor to form a toner image, a transferring process of the developed toner image onto a recording medium, and a fixing process of the transferred toner image. FIG. 1 is a schematic diagram showing an image data writing optical system in a laser printer. Referring to FIG. 1, the above processes are described in detail.
Image data are converted into laser beams having intensity of several steps via an image data processing section 41, a laser driving circuit 43, and a light source 22e. The diameter of a laser beam is converted into a diameter suitable to be transmitted via lenses 24e and 28d, and a mirror 30. Further, the laser beam is scanned by a polygon mirror 32. The laser beam output from the polygon mirror 32 is input to a fθ lens 34d and the laser beam is formed so that the image height (measured in the width direction of the fθ lens 34d) is linearly changed for a scanning angle by the fθ lens 34d. The diameter of the laser beam is converted into a diameter suitable for writing on a photoconductor 14 by a cylindrical lens 36d and the laser beam is irradiated onto the photoconductor 14.
An electric charge is generated on the photoconductor 14 corresponding to the amount of the laser beam. Therefore, when toners having a reverse electric charge come in contact with the photoconductor 14, the toners corresponding to the amount of the laser beam are adhered to the photoconductor 14 and a toner image is developed. When a paper which is charged more greatly than the photoconductor 14 approaches the photoconductor 14, the toner image is transferred to the paper. Since the toner image transferred on the paper is only put on the paper, the toner image is fixed on the paper by applying heat or pressure to the toner and the paper.
In the above processes, a so-called out of color registration such as skewing of an image, a bow-shaped distortion in an image, and a change of a scale factor of an image may occur, due to insufficient accuracy of components of which the image data writing optical system is composed and errors when the components are assembled.
The skewing of the image is caused by a difference between the scanning direction of a laser beam and the main scanning direction on the photoconductor 14. FIG. 2 is a schematic diagram illustrating the skewing of the image. In FIG. 2, on the photoconductor 14, the continuous arrow line is the scanning direction originally desired to write a laser beam and the dotted arrow line is the actual scanning direction of the laser beam which causes the skewing. The inclination of the dotted arrow line is caused by errors when installing lenses and insufficient accuracy of the surface of the polygon mirror 32. The bow-shaped distortion of the image is caused by deforming of the photoconductor 14 to be bow-shaped or bending of a paper on which the image is formed. The change of the scale factor of the image is caused by insufficient accuracy of lens characteristics.
When the out of color registration occurs, the image is distorted and the image quality is degraded. In order to prevent occurrence of the out of color registration, high accuracy is required in manufacturing various components and installing them; consequently, the cost is increased. Therefore, a technology is desired in which technology the out of color registration is restrained and the cost is not increased while maintaining high quality of images.
In Japanese Laid-Open Patent Application No. 8-146317 and Japanese Patent No. 2749367, the out of color registration is corrected by changing an optical path of a laser beam which is used to form an image on a photoconductor.
However, in order to realize the technologies in Japanese Laid-Open Patent Application No. 8-146317 and Japanese Patent No. 2749367, it is required that specific hardware for correcting the out of color registration be disposed in an optical system of a printer. Therefore, the size of the apparatus may become large and the cost may be increased. In addition, since the correction of the skewing and the correction of the bow-shaped distortion mutually influence therebetween, when one distortion is corrected, the other distortion is increased. Further, since the correction is executed by hardware, accuracy and performance beyond the characteristics of the hardware cannot be realized.
In Japanese Laid-Open Patent Application No. 10-62700, a technology is disclosed in which technology the scale factor is corrected by modulating a video clock which is used for forming an image on a photoconductor.
Further, in Japanese Laid-Open Patent Application No. 8-85236 and Japanese Laid-Open Patent Application No. 2000-112206, methods are disclosed in which methods the out of color registration at the time of color printing is corrected by image processing. However, in a printer which executes printing by using a pseudo continuous tone process, when the method is applied to image data after the pseudo continuous tone process is applied, a pixel used for the correction of the scale factor interferes with a spatial frequency of the pseudo continuous tone which is used for printing; consequently, interference fringes appear on an output image. In order to solve this problem, in Japanese Laid-Open Patent Application No. 2003-274143, correction is executed so that interference does not occur after the pseudo continuous tone process.
However, in Japanese Laid-Open Patent Application No. 2003-274143, when the correction is executed after executing the pseudo continuous tone process, the correction is limited in some cases. Especially, the limitation can be a great problem in a case where the resolution does not change before and after executing the pseudo continuous tone process. One example is described below.
In the example, a case is studied in which an image larger than an original image is output due to an error of the scale factor. When the correction of the scale factor is executed after executing the pseudo continuous tone process, one pixel must be thinned out from several pixels. A case is described in which pixels at specific positions are simply thinned out in the main scanning direction. FIG. 3 is a diagram showing pixels of an image in a pixel thinning out process. In FIG. 3, a rectangle is a pixel and black pixels are to be thinned out. As shown in FIG. 3(a), when a black pixel overlaps a thin line, the thin line vanishes.
In Japanese Laid-Open Patent Application No. 2003-274143, in order to solve this problem, as shown in FIG. 3(b), the positions of the pixels to be thinned out are shifted among lines in the main scanning direction. With this, all the thin lines are prevented from vanishing. However, in this case, the thin lines partially vanish and some original continuous lines become dashed lines.